The present invention relates to a process for the purification of vinyl-ACA in a mixture of vinyl-ACA and 7-ADCA, by depletion of 7-ADCA in a mixture of vinyl-ACA and 7-ADCA.
Vinyl-ACA of formula 
may be used as intermediate in the production of highly active, oral antibiotics, e.g. cefixime and cefdinir of formulae 
Vinyl-ACA may be produced, for example, by Wittig reaction of a corresponding cephalosporin-3-ylide, which may have the amine group and the carboxylic acid attached to the ring system protected, with formaldehyde (see e.g. Journal of Antibiotics, Vol. 38, No. 12, 1739 ff; or EP-0 503 453; or EP-0 597 429). We have found that such vinyl-ACA may be contaminated, e.g. by 7-ADCA. This is consistent with the fact that phosphine alkylenes (ylides) or quaternary phosphonium compounds can hydrolyse to form the corresponding alkane and phosphine oxide (see e.g. Houben Weyl, Methoden der organischen Chemic, Phosphorverbindungen I, volume 12/1, especially pages 108 and 119). We found accordingly, when producing a compound of formula I via a Wittig reaction, as a by-product 7-ADCA of formula 
or a protected derivative thereof may be formed. Furthermore, if 7-ACA is produced via fermentative production of cephalosporin C and subsequent conversion to 7-ACA, the thus formed 7-ACA may contain 7-ADCA, because the 7-ADCA-analogous cephalosporin is formed in the course of fermentation, or is not wholly metabolised to cephalosporin C. Thus, 7-ACA, used for example as a starting material for 7-ACA, may often have an undesired 7-ADCA content, for example of more than 1%.
In the production of active cephalosporins, e.g. cefixime and cefdinir, wherein an intermediate of formula I may be used, the 7-ADCA content in vinyl-ACA of formula I should be as low as possible, because upon appropriate further substitution of a compound of formula I, 7-ADCA could react in the same way as vinyl-ACA which would result in contamination of the desired active vinyl-ACA compounds, e.g. cefixime or cefdinir, by analoguously substituted 7-ADCA-compounds, which are difficult to separate.
According to the present invention, vinyl-ACA in a mixture of vinyl-ACA and 7-ADCA can be purified in an economical manner by depletion of 7-ADCA to, e.g. less than 0.1% to 0.8%, such as less than 0.1% to 0.6%, for example 0.3% to 0.8%. This is remarkable, because 7-ADCA and vinyl-ACA are chemically very similar compounds.
In one aspect the present invention provides a process for the depletion of 7-ADCA of formula II in a mixture of 7-ADCA and vinyl-ACA of formula I, preferably by a process, wherein
a) a mixture of a salt of a compound of formula I and a compound of formula II is subjected to crystallization, the crystallised salt is isolated and converted into a compound of formula I, containing less compound of formula II than the mixture of a compound of formula I and formula II, or
b) a mixture of a compound of formula I and a compound of formula II is subjected to chromatography.
The salt includes for example a cationic salt of the carboxylic acid group and an amine salt of the carboxylic acid group in a compound of formulae I and II.
In a further aspect the present invention provides a process as described above, wherein a mixture of a salt of a compound of formula I and a compound of formula II is a mixture of compounds of formulae 
wherein X+ denotes a cation, or a compound of formula 
wherein R1, R2 and R3 are the same or different and independently of one another denote hydrogen, alkyl, aryl, aralkyl, or cycloalkyl; or
R1 and R2 together with the nitrogen atom form a heterocycle and R3 is as defined above.
The cation includes a cation of the alkali series, for example Li+, K+, Na+.
Preferably R1 denotes hydrogen and R2 and R3 independently from one another denote alkyl or aralkyl. R1 and R2 together with the nitrogen atom may denote a heterocycle, preferably a 5- or 6 membered heterocycle, having for example 1 to 3 heteroatoms.
If not otherwise defined herein, any carbon containing radical contains up to 10 carbon atoms. Alkyl includes straight chain or branched C1-22alkyl, preferably C1-12alkyl, such as C1-8alkyl. Aryl includes unsubstituted aryl or substituted aryl, preferably phenyl or, mono- or polysubstituted phenyl. Aralkyl includes unsubstituted aralkyl, or substituted aralkyl, for example benzyl. Cycloalkyl includes C3-8cycloalkyl, such as C3-6cycloalkyl. A heterocycle includes unsubstituted heterocycle or substituted heterocycle, for example 5- or 6-membered heterocycle. A heterocycle may contain one or several heteroatoms, for example N, S, O. Substituents of any aryl group, aralkyl group and of any heterocycle include substitutents which are inert under the corresponding reaction conditions, for example alkyl, aryl, alkoxy, aryloxy, halogen, nitro, optionally protected amine groups, optionally protected hydroxy.
Process variant a) may be carried out as follows:
A salt of a mixture of a compound of formula I and of formula II may be produced, for example, by adding a salt forming agent to a mixture of a 7-ADCA and vinyl-ACA in a solvent. A salt forming agent includes, for example, a base. A base includes, e.g. an inorganic base, e.g. a hydroxide, for example an alkali hydroxide; and a salt having a cation source; such as an inorganic salt, for example an alkali salt, such as a carbonate, hydrogencarbonate; and an organic salt, for example the salt of a carboxylic acid, for example an alkali salt, of, for example acetic acid or 2-ethylhexanoic acid; and an organic base, for example a nitrogen base, for example ammonia or an amine, for example an amine of formula 
wherein R1, R2 and R3 are as defined above.
In a further aspect the present invention provides a process as described above, wherein a mixture of a salt of a compound of formula I and a compound of formula II is produced by addition of a salt forming agent to a mixture of a compound of formula I as defined in claim 1 and a compound of formula II as defined in claim 1 in a solvent, preferably a process, wherein the salt forming agent is an inorganic base, an inorganic salt, an organic salt or a nitrogen base; preferably, the inorganic base is a hydroxide; the inorganic salt is an inorganic alkali salt; the organic salt is an alkali salt of a carboxylic acid; and the nitrogen base is a compound of formula 
wherein R1, R2 and R3 are as defined above.
A solvent includes an aprotic solvent and a protic solvent, for example an amide, such as N,N-dimethylformamide, a ketone, such as acetone; an alcohol, such as methanol, ethanol or one of the isomeric propanols or butanols, for example isopropanol; a nitrile such as acetonitrile; ethers or chlorinated hydrocarbons; water; and mixtures of solvents.
In one aspect a mixture of vinyl-ACA and 7-ADCA may be dissolved in water or in an aqueous organic solvent, for example a mixture of water and a ketone; and a mixture of water and an alcohol, such as ethanol or isopropanol; in the presence of a salt forming agent. The pH may be appropriately adjusted, for example by addition of a base, having, for example, a Li, Na or K-source, such as an acetate; particularly in case that an organic base, such as a nitrogen base is used as salt forming agent. An anti-solvent, for example a nitrile, such as acetonitrile; an alcohol such as methanol, ethanol or one of the isomeric propanols or butanols; an ether, such as diethyl ether, tetrahydrofuran or tert.butylmethyl ether; a ketone, such as acetone; or an ester, such as ethyl acetate or acetic acid isopropyl ester; or mixtures of anti-solvent; may be added. The salt of vinyl-ACA, or a mixture of a salt of vinyl-ACA and 7-ADCA, wherein the 7-ADCA content is less than in the mixture used for salt production, may crystallize.
In another aspect a mixture of vinyl-ACA and 7-ADCA may be suspended in a practically water-free organic solvent, such as an amide, a ketone; an alcohol; a nitrile; an ether; a chlorinated hydrocarbon; and mixtures of water-free organic solvent. Preferred solvent include a mixture of methanol with a ketone or a higher alcohol. A nitrogen base, for example of formula VI, and an anti-solvent as defined above may be added.
Surprisingly the undesired salt of 7-ADCA may be better soluble than the desired salt of vinyl-ACA and a salt of vinyl-ACA or a mixture of a salt of vinyl-ACA and 7-ADCA in which the content of 7-ADCA is less than in the mixture used for salt production is precipitated. By isolation of the precipitate which may be carried out as conventional, separation of a compound of formula III and of a compound of formula IV may be effected.
If required, process variant a) may be repeated with a product obtained according to process variant a) resulting in further depletion of the undesired compound of formula II. A mixture of a salt of vinyl-ACA and 7-ADCA obtained may be resuspended in the solvent system in which crystallisation had been carried out, and the solubility product may be adjusted, for example by addition of solvent or anti-solvent as appropriate, with the effect of further depletion of 7-ADCA.
A compound of formula III in crystalline form is new.
In another aspect the present invention provides a compound of formula 
wherein X+ is as defined above, in crystalline form.
In another aspect the present invention provides the
dicyclohexylammonium salt of 7-amino-3-vinyl-cephalosporanic acid in crystalline form;
tert.octylammonium salt of 7-amino-3-vinyl-cephalosporanic acid in crystalline form;
N-benzyl-tert.butylammonium salt of 7-amino-3-vinyl-cephalosporanic acid in crystalline form;
2-ethyl-1-hexylammonium salt of 7-amino-3-vinyl-cephalosporanic acid in crystalline form;
potassium salt of 7-amino-3-vinyl-cephalosporanic acid in crystalline form.
The salt of vinyl-ACA or of the mixture of vinyl-ACA and 7-ADCA may be isolated and converted into the free compound of formula I and formula II as conventional, for example by treatment with an acid, such as an inorganic acid, such as hydrochloric acid, sulphuric acid, preferably sulphuric acid; or an organic acid such as an organic carboxylic acid.
Process variant b) according to the present invention may be carried out as follows:
Chromatography preferably may be adsorption chromatography.
A mixture of vinyl-ACA and 7-ADCA may be dissolved, for example in water, for example in the presence of a base, e.g. ammonia. This solution is subjected to chromatography using an adsorbent. An adsorbent includes activated charcoal, e.g. Norit CG-1 or Cecarbon GAC 40; or an adsorber resin, such as styrene-divinylbenzene copolymerisates, for example Dianion HP 20 (CAS No. 55353-13-4), Dianion HP 21 (CAS No. 92529-04-9) or Dianion SP 207 (CAS No. 98225-81-1) from Mitsubishi Kasei Corporation; Amberlite XAD 1180 (CAS No. 97396-56-0), Amberlite XAD 1600 (CAS No. 153796-66-8) or Amberlite XAD 16 (CAS No. 102419-63-8) from Rohm and Haas or Amberchrom CG 161 (CAS No. 131688-63-6) from TosoHaas; preferably CG 161 and XAD-1600. Elution may be carried out with water. The compound which elutes earliest may be in general 7-ADCA. Thus, fractions of the compound of formula II, mixtures of the compound of formula I and II and the pure compound of formula I may be obtained. Isolation of a compound of formula I may be carried out by adjustment of the pH of a fraction containing a compound of formula I obtained by the present invention to around the isoelectric point of a compound of formula I, for example as conventional, e.g. by addition of an acid, such as an inorganic acid, for example hydrochloric acid. The compound of formula I may crystallise.
Process variant b) is very simple to carry out. Elution may be effected with a purely aqueous medium, no organic solvent is to be used. The equipment required is simple. We have found that there is no need for elution by means of a gradient, nor step-wise elution, nor pH changes in the course of chromatographic purification.
Process variant a) can be combined with process variant b) for still more effective depletion of 7-ADCA in a mixture of vinyl-ACA and 7-ADCA.
In another aspect the present invention provides the use of a compound of formula 
wherein X+ is as defined above, or the use of a process for the depletion of 7-ADCA of formula II in a mixture of 7-ADCA and vinyl-ACA of formula I, in the production of highly active cephalosporins, for example cefixime and cefdinir.
Processes a) and b) of the present invention represent very economical methods of separating 7-ADCA from mixtures of vinyl-ACA with 7-ADCA, which are very simple to carry out and which are suitable for use on industrial scale.
In another aspect the present invention provides a depletion process of 7-ADCA of formula 
in mixtures of vinyl-ACA of formula 
with 7-ADCA, characterised in that
a) a mixture of vinyl-ACA and 7-ADCA is converted into salts of formula 
wherein X⊕ is Li⊕, Na⊕, K⊕ or a cation of formula 
wherein R1xe2x80x2, R2xe2x80x2 and R3xe2x80x2 are the same or different and independently of one another denote hydrogen, (C1-8)alkyl, optionally substituted benzyl or phenyl or (C4-8)cycloalkyl, or R1xe2x80x2 and R2xe2x80x2 together with the nitrogen form a 5- or 6-membered heterocycle which optionally contains a further one or two hetero atoms, and R3xe2x80x2 is as defined above, by reaction of the mixture of vinyl-ACA and 7-ADCA with a lithium, sodium or potassium base or with an amine of formula 
wherein R1xe2x80x2, R2xe2x80x2 and R3xe2x80x2 are as defined above, whereby
xcex1) the reaction is carried out in a solvent or solvent mixture in which the salts of formulae III and IV have different solubilities, or
xcex2) the salts of the compounds of formulae III and IV are suspended in a solvent or solvent mixture, and the solubility product is adjusted,
and after isolating the compound of formula III, this is converted using an acid into the compound of formula I having no content or a reduced content of 7-ADCA, or
b) a solution of a mixture of vinyl-ACA with 7-ADCA is chromatographed.
In the following examples all temperatures are given in degrees celsius.
The following abbreviations are used: